Saturday, 1 May 2010

More dark entries

I have another bucketful of dark matter news and gossips, some market fresh, some long overdue. Let me bullet it out, even if each may deserve a separate post.

The Xenon100 experiment in Gran Sasso - currently the most sensitive dark matter detection experiment on Earth - is up and running. The results from a short 11 days run in November last year were presented at the WONDER2010 conference a month ago. The signal region where nuclear recoils are supposed to appear is below the blue line. As you can see, bastards really have zero background events. Even this small amount of data allows them to set the limits on the dark matter - nucleon cross section comparable to those obtained by CDMS after many months of running. The experiment is continuously taking data since January and the plan is to run for an entire year. As of today they have roughly 10 times more data on tape, but it's not yet clear when the new chunk will be unblinded and analyzed. Can't wait.

Xenon100 can take their time because direct competitors are falling like flies. LUX, a US based experiment that relies on practically the same technology, is stranded until at least next year waiting for their underground cavern to be ready. WARP, a similar experiment next door in Gran Sasso but filled with argon rather than xenon as the target, was aborted last year due to an electrical failure. The latest (unconfirmed) rumor is that XMASS - a 1 ton xenon dark matter experiment in Japan - has been downed due to a simple engineering error. New York City psychics whisper in terror about dark ectoplasm currents sourced somewhere in northern Manhattan.

Back to Gran Sasso. CRESST's presentation at WONDER2010 devoted 1 slide to wild speculations about their latest unpublished results on dark matter detection. CRESST uses CaWO4 crystals as the target using and detect scintillation light and phonons to sort out the signal of dark matter recoiling on the nuclei making the crystal. The cool thing about the experiment is that using the light-to-phonon ratio they can to some extent tell whether a nuclear recoil occurred on tungsten or on oxygen. In the tungsten (blue) band, where weak scale dark matter is expected to show up first, there is almost no events. But in the oxygen band (reddish) there is something weird going on. Of course, most likely this is some sort of background that the collaboration has not pinned down yet. But another possible interpretation is that the dark matter particle is very light so that it bounces off heavy tungsten nuclei but still can give a kick to much lighter oxygen nuclei. Furthermore, the slide mentions that the event rate in the oxygen band displays a hint of annual modulation expected from dark matter scattering. Curiouser and curiouser...

...especially if CRESST data are viewed from a somewhat different angle. Juan Collar, apart from being a guest-blogger, has a daytime job at CoGeNT - another dark matter experiment that has recently seen hints of light dark matter particles. A few weeks ago during a workshop in New York Juan flashed the following plot (Content Warning:the plot below makes respectable physicists shout obscenities):These are the CRESST data from the tungsten band plotted as the differential recoil spectrum. Naively, the spectrum fits the one expected from light dark matter particles of mass approximately 10 GeV, that is the same ballpark that also fits the CoGeNT data!

The situation could be clarified by the CDMS experiment. Although they finished data-taking, they are sitting on a large amount of data collected by their silicon detectors, of which only a part was analyzed and made public (their most recently published limits are based on data from the germanium detectors). Silicon is a fairly light element (A=28) and therefore it is more suitable than germanium for studying light dark matter. Thus CDMS has the potential to exclude the light dark matter interpretation of the CoGeNT and CRESST signals; unfortunately this does not seems to be their priority right now. CRESST itself should release a full-fledged analysis of their data soon, which should provide us with more solid information. However, CRESST at this point is not a background free experiment. Therefore in the nearest future we should expect a wilderness of mirrors rather than clear-cut answers. In other words, more rumors ahead :-)

Update: The paper with first Xenon100 results is now out on arXiv. The analysis chalenges the dark matter interpretation of the CoGeNT data. As you can see on the plot, the region of the parameter space favored by CoGeNT is excluded by Xenon100 at 90% confidence level. One should however note that these limits strongly depend on the quenching factor in xenon (that is how much of recoil energy gets converted into light). Different experimental measurements of that quenching factor point to different trends at low recoil energies (see fig.1 in the Xenon100 paper), which leaves some wiggle room.

Update #2: Just 2 days later Xenon100 gets a smackdown. A new paper by Collar and McKinsey casts doubt whether Xenon100 has any sensitivity to light dark matter particles consistent with the CoGeNT signal. As already hinted, Xenon100's assumptions about the quenching factor at low energies are controversial. Another assumption that is questioned concerns the distribution of the number of photoelectrons near threshold:

...limits depend critically (...) on the assumption of a Poisson tail in the modest number of photoelectrons that would be generated by a light-mass WIMP above detection threshold (...). We question the wisdom of this approach when the mechanisms behind the generation of any significant amount of scintillation are still unknown and may simply be absent at the few keVr level. To put it bluntly, this is the equivalent of expecting something out of nothing.

23 comments:

Chris
said...

On Juan Collar's plot, did you mean the CRESST data from the *oxygen* band?

Juan's plot extends to energies below 10 keV (where CRESST put the lower cut on their signal region) and there is quite a lot of events there, see the plot above. In any case, this is just funny games; any solid statement about these events can only come from CRESST.

I find it truly hard to believe that people take this shit seriously - that they are doing local experiments to test an idea that depends on GR, a theory tested only in the vicinity of Earth and Sun, in a vacuum, being correct when applied to the entire Universe in its current form. It's unbelievable how much the culture of science has declined.

Yes I have looked. I haven't yet checked myself what happens when you use the fit to Manzur et al. data instead (definitely the limits will be relaxed, but the question is by how much). I will look at it as soon as I have a minute; we can skype about it tmw.

There are two types of mistakes in the XENON paper: possible bias etc., and then what may amount to scientific misconduct: the text and fig. 1 look to me tailored to mislead. The footnote in the "Comments" (now that it can be understood) carries more weight than the rest of it. This has the potential to get uglier really fast.

I have no idea how seriously to take this, but for even more fun, look at John Ralston's talk at Pheno, which claims many experiments have badly misestimated their neutron backgrounds. It would be nice to hear the experiments' response to this dramatic claim.

I don't see the Collar & McKinsey paper artfully walking a fine line between snark and professionalism -- more like stomping both sides of the line with hobnail boots. ("In a fascinating attempt at trompe l'oeil....")

Yikes! You're right, is there a coherent though in Ralston's talk? It seems to be lacking a clear understanding of the physics. For one, no matter how much neutrons misbehave at low energies, they cannot be a background if they have all been removed with shielding and overburden. For another, several calibrations of Leff have been performed with neutrons at energies where they are well behaved and well understood. There is a deep literature on the subject...sheesh.

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The main goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...